Answer:
Mainly because if a disorder is dominant, then it will be manifested by all who have the gene. If the disorder is severe, then it will reduce the chances of the individual surviving to grow up and reproduce (unless the disorder only manifests symptoms in later life). If they do reproduce, then 50% of their offspring will inherit the gene, and be at a similar disadvantage in reproduction. Thus, there is likely to be selection against the gene. By contrast, a recessive disorder is only manifested if both parents pass on the abnormal gene, and an individual with one gene for the disorder is usually at no disadvantage. Thus, there is much less selection against abnormal recessive disorders.